The present invention relates to optoelectronic devices such as light-emitting diodes.
Light emitting diodes or xe2x80x9cLEDsxe2x80x9d include thin layers of semiconductor material of two opposite conductivity types, referred to as p-type and n-type. The layers are disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at the other end of the stack. For example, the various layers may be deposited in sequence on a substrate to form a wafer. The wafer is then cut apart to form individual dies which constitute separate LEDs. The junction between the p-type and n-type material may include directly abutting p-type and n-type layers, or may include one or more intermediate layers which may be of any conductivity type or which may have no distinct conductivity type. In operation, electric current passing through the diode is carried principally by electrons in the n-type layers and by electron vacancies or xe2x80x9cholesxe2x80x9d in the p-type layers. The electrons and holes move in opposite directions toward the junction, and recombine with one another at the junction. Energy released by electron-hole recombination is emitted as light. As used in this disclosure, the term xe2x80x9clightxe2x80x9d radiation includes infrared and ultraviolet wavelength range, as well as the visible range. The wavelength of the light depends on factors including the composition of the semiconductor materials and the structure of the junction.
Electrodes are connected to the n-type and p-type layers near the top and bottom of the stack. The materials in the electrodes are selected to provide low-resistance interfaces with the semiconductor materials. The electrodes, in turn, are provided with pads suitable for connection to wires or other conductors which carry current from external sources. The pad associated with each electrode may be a part of the electrode, having the same composition and thickness of the electrode, or may be a distinct structure which differs in thickness, composition, or both from the electrode itself. The term xe2x80x9celectrode-pad unitxe2x80x9d is used in this disclosure to refer to the electrode and pad, regardless of whether the pad is a separate structure or merely a region of the electrode.
Some LEDs have electrodes on the bottom surface of the bottom semiconductor layer. For example, the various layers may be deposited in sequence on an electrically conductive substrate, and the substrate may be left in place on the bottom surface to act as a bottom electrode. However, LEDs formed from certain semiconductor materials normally use nonconductive substrates to promote proper formation of the semiconductor layers. The nonconductive substrate typically is left in place, so that an electrode cannot be provided on the bottom surface of the bottom layer. For example, gallium nitride-based materials such as GaN, AlGaN, InGaN and AlInGaN are used to form LEDs emitting light in various wavelength ranges including blue and ultraviolet. These materials typically are grown on insulating substrates such as sapphire or alumina.
LEDs incorporating an insulating substrate must include a bottom electrode at a location on the stack above the substrate but below the junction. Typically, the upper layer or layers of the stack are removed in a region covering part of the area of each die after formation of the stack, so as to provide an upwardly-facing lower electrode surface on a layer at or near the middle of the stack in each die. This leaves a region referred to as a xe2x80x9cmesaxe2x80x9d projecting upwardly from the lower electrode surface and covering the remaining area of the die. The area of the die occupied by the lower electrode surface does not emit light. It is desirable to keep the horizontal extent of this inactive area as small as possible.
The top electrode typically is formed on the top surface of the stack, i.e., the top surface of the top semiconductor layer. Typically, the layers in the stack above the junction are transparent, so that light emitted at the junction can pass out of the stack through the top surface. The top electrode is arranged so that it does not block all of the emitted light. For example, an opaque top electrode may cover only a small portion of the top surface of each die. However, the current passing from such an electrode will tend to flow downwardly through the stack so that the current passes predominantly through the area of the junction disposed beneath the electrode. This phenomenon, referred to as xe2x80x9ccurrent crowdingxe2x80x9d, results in light emission concentrated in that area of the junction beneath the electrode, precisely where it will be most effectively blocked by the electrode. The amount of useful light reaching the outside of the die per unit of electrical current passing through the die, commonly stated as the external quantum efficiency of the die, is reduced by this phenomenon. Current crowding can also occur in the lower region, so that light emission is concentrated in the area of the junction near the lower electrode. Current crowding is a significant consideration with LEDs formed from materials having relatively high electrical resistivity, such as the gallium nitride-based materials.
To alleviate the current crowding problem, LEDs have been provided with transparent top electrodes, formed from thin layers of metals and metal compounds. A pad, which is typically opaque, occupies a small portion of the top surface. The transparent top electrode spreads the current in horizontal directions from the pad, so that current flow down through the stack is spread more evenly over the horizontal extent of the mesa. However, the top electrode normally must be quite thin in order to make it transparent and minimize the amount of light absorbed by the electrode. Therefore, the transparent electrode typically has appreciable resistance to current flow in the horizontal directions. There may still be significant current crowding in the area beneath the pad of the top electrode.
U.S. Pat. No. 5,563,422 suggests placing the pad of a top transparent electrode at one corner of a square die and forming the lower pad region by removing the upper portion of the stack at the diagonally opposite corner of the die. This assertedly results in a relatively even current distribution over the horizontal extent of the mesa. However, there are still needs for further improvements and alternative arrangements.
The present invention addresses these needs.
One aspect of the invention provides a light-emitting diode which includes a stacked structure. The stacked structure incorporates a first region of a first conductivity type, a second region of a second conductivity type and a light-emitting p-n junction between these regions. The stacked structure defines a lower contact surface and a mesa projecting upwardly from the lower contact surface. The first-type region is disposed in the mesa and defines a top surface of the mesa. The second-type region defines the lower contact surface. In a light-emitting diode according to this aspect of the invention, the lower contact surface substantially surrounds the mesa. The diode desirably includes a lower electrode-pad unit incorporating a lower electrode in contact with the lower contact surface. Most preferably, the lower electrode substantially surrounds the mesa. A top electrode-pad unit incorporates a top pad overlying only a portion of the top surface of the mesa.
As further discussed below, the lower electrode substantially surrounding the mesa promotes current spreading from the top pad in substantially all horizontal directions, and thus promotes more uniform current distribution at the junction.
Preferably, the top electrode-pad unit includes a transparent top electrode overlying at least a major portion of the top surface of the mesa, the transparent electrode being in contact with the first-type region at said top surface. The top pad desirably covers only a small portion of the transparent electrode. In a particularly preferred arrangement, the top pad is disposed adjacent the center of the top surface of the mesa.
A further aspect of the invention provides a diode including a stack structure similar to that discussed above, but has a lower electrode-pad unit including a lower electrode generally in the form of an elongated strip with its length extending in a first horizontal direction. For example, the lower contact surface may be in the form of a ledge extending adjacent to one edge of the mesa, and the lower electrode may extend lengthwise along this ledge. The diode according to this aspect of the invention desirably includes a top electrode-pad unit incorporating a top pad. The top pad overlies only a small portion of said mesa top surface. Preferably, the top pad is in the form of a spot, and has an extent in the first horizontal direction substantially less than the length of the lower electrode. In a particularly preferred arrangement, the top pad is disposed adjacent to the edge of the mesa furthest from the lower electrode. This top pad may be aligned with the midpoint of the lower electrode. For example, in a rectangular die, the lower electrode may extend parallel to one edge of the mesa, along substantially the entire length of that edge, whereas the top pad may be disposed adjacent the opposite edge of the mesa, near the midpoint of such edge. Dies according to this aspect of the invention promote current spreading, while using a lower contact surface which does not occupy a large portion of the die area.
A die according to a further aspect of the invention also includes a stack structure similar to those discussed above. However, the die according to this aspect of the invention has an indentation in the mesa at one edge, referred to as the near edge, the indentation being disposed adjacent the midpoint of the near edge and extending downwardly from the top surface to the lower contact surface, so that the floor of the indentation forms part or all of the lower contact surface. In a die according to this aspect of the invention, the top pad overlying only a portion of said top surface adjacent the midpoint of an edge opposite from the near edge, referred to as the far edge. The lower electrode-pad unit includes a lower pad having an extent in the direction of the near edge substantially shorter than said near edge. The lower pad is disposed at least partially in the indentation, and most preferably is disposed entirely within the indentation. The die according to this aspect of the invention minimizes the area occupied by the lower contact surface.